1. Field of the Invention
This invention relates to methods for forming an electrodeposited film or an electrode over the surface of an object to be treated using an ultra-short pulse laser.
2. Description of the Related Art
Attempted applications of laser to electrode formation technology can be broadly classified into plating and etching.
The basic principle of application to plating is that an object to be plated soaked in a plating solution is irradiated with a laser beam and the resultant thermo-electromotive force causes part of the object to be plated (see, for instance, Reference 1: specification of the U.S. Pat. No. 4,349,583, and Reference 2: IBMJ. Res. Develop., Vol. 42, No.5, September 1998).
According to this plating method, a bias voltage may or may not be applied from an external circuit. Application of a bias voltage would result in electrolytic plating, which allows partial plating as the laser-irradiated part is plated faster than the non-irradiated part. Non-application of a bias voltage would result in non-electrolytic plating. In this case, the laser-irradiated part functions as the active electrode and the non-irradiated part, as the opposite electrode, and the plating of the irradiated part and the dissolution (etching) of the non-irradiated part take place at the same time.
The above-cited specification of the U.S. Pat. No. 4,349,583, for example, discloses a case of non-electrolytic plating. Referring to this prior art, FIG. 13 illustrates a case in which a less noble metal (LNM) substrate disposed in a plating bath is irradiated with a laser beam LB and a more noble metal (MNM) electrode is formed by metal-plating over the LNM substrate, while FIG. 14 shows an instance in which an LNM film over a glass substrate G is irradiated with the laser beam LB and an MNM electrode is formed by metal-plating over the LNM film.
Since FIG. 13 and FIG. 14 illustrate cases non-electrolytic plating involving no application of a bias voltage, in both cases LNM portions around the MNM part are etched simultaneously with the formation of metal-plating as an MNM electrode (see the ADR portion in FIG. 13 and the L portion in FIG. 14).
As applications of the above-described plating technique using laser irradiation, maskless patterning and electrode repairing are proposed.
Incidentally, the laser so far considered for application to plating is either a continuous beam (in the case of the U.S. patent of Reference 1), such as an argon laser, or a pulse laser, whose time width is tens of picoseconds or more, such as a YAG laser.
Since these lasers, in order to obtain a sufficient thermo-electromotive force, require a high output of 102 to 106 W/cm2 in the intensity of irradiating light, plating over a large area needs either a high output laser or long duration of irradiation.
There is another problem in the conventional application to plating that, since it is a thermal process, thermal diffusion gives rise to overhangs on the edges of plating as illustrated in FIG. 13 and FIG. 14.
Furthermore, for electrode formation, in many cases a hole is first bored by etching followed by electrode formation (plating) in the bored part, but since no sharply edged hole can be bored by similar thermal diffusion with a continuous beam and a pulse of tens of picoseconds in time width, different lasers need to be used for etching and electrode formation.
In view of these problems, this invention is intended to provide methods which permit formation of efficient electrodes consuming less energy and excelling in quality.